Patellar clunk is a rare complication of knee replacement surgery. It's more common in older design TKR and folks with greater knee mobility.

A nodule develops behind the kneecap and gets stuck in a notch against the thigh bone, "clunking" or "crunching" everytime the knee is bent. The nodule can be removed arthroscopically. http://www.nlm.nih.gov/medlineplus/ency/article/002972.htm